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Abstract

Understanding the optical properties of clustered quantum dots (QDs) is essential to the design of QD-based optical phantoms for molecular imaging. Single and clustered core/shell colloidal QDs of dimers, trimers, and tetramers are self-assembled, separated, and preferentially collected using electrospray differential mobility analysis (ES-DMA) with electrostatic deposition. Multimodal optical characterization and analysis of their dynamical photoluminescence (PL) properties enables the long-term evaluation of the physicochemical and optical properties of QDs in a single or a clustered state. A multimodal time-correlated spectroscopic confocal microscope capable of simultaneously measuring the time evolution of PL intensity fluctuation, PL lifetime, and emission spectra reveals the long-term dynamic optical properties of interacting QDs in individual dimeric clusters of QDs. This new method will benefit research into the quantitative interpretation of fluorescence intensity and lifetime results in QD-based molecular imaging techniques. The process of photooxidation leads to coupling of the QDs in a dimer, leading to unique optical properties when compared to an isolated QD. These results guide the design and evaluation of QD-based phantom materials for the validation of the PL measurements for quantitative molecular imaging of biological samples labeled with QD probes.

TEM images of QD clusters and their size distributions following DMA separation. TEM images of (a) monomeric, (b) dimeric, (c) trimeric, and (d) tetrameric QDs collected on TEM grids using the ES-DMA technique. (e) Distribution of single QDs and clustered QDs after ES-DMA assembly/screening with Gaussian fits for the distribution of each population. The averaged length and width of the each single QD including shell and functionalized coating is about 11 nm and 4 nm, respectively.

Spectral diffusion of single QDs in the cluster. Typical time evolution of the PL spectrum of (a) an isolated QD, (b) an isolated dimeric cluster, and (c) an isolated trimeric cluster. For clarity, the PL spectrum taken at each time point is normalized in order that all the maxima of all the spectra are of the same value.

Simultaneously measured multiple dynamic PL characteristics of a single QD and a dimeric cluster. (a) PL intensity fluctuations of the PL emission photon counts with a bin time of 10 ms. The signal is colored according to the fitted peak PL emission wavelength as denoted in the color bar, with black used for signals below the fitting threshold. A histogram of this intensity fluctuation is presented on the right side of the plot; (b) spectrum vs. time; (c) PL lifetime vs. time where the data points with arrows denote anomalous increased lifetimes. (d-f) Dynamic PL characteristics of a dimeric QD cluster comparable to results from the single QD (see panels a-c). The red dotted lines are visual guides for the time period of photooxidation.

Quantitative analysis of the dynamic PL characteristics. (a) PL lifetimes vs. PL intensities and (b) the on-time probability plot analyzed from two different time periods of the single QD data set (section (i), and section (ii) shown in Fig. 5(a), corresponding to before and during the photooxidation period, respectively). (c, d) Same plots from three different time periods of the dimeric QD data set (sections (i), (ii), and (iii) in Fig. 5(d), corresponding to before, during, and after the photooxidation period, respectively). In panels (a) and (c), mean and standard deviation are displayed over scatter plots.